The present invention is generally related to analytical tools for the biological and chemical sciences, and in a particular embodiment, provides microfluidic devices, systems, and methods for determining the viscosity of fluids within microfluidic channels of a microfluidic network, optionally without adding dye (or other agents) which can alter the properties of the fluids.
Microfluidic systems are now in use for the acquisition of chemical and biological information. These microfluidic systems are often fabricated using techniques commonly associated with the semiconductor electronics industry, such as photolithography, wet chemical etching, and the like. As used herein, xe2x80x9cmicrofluidicxe2x80x9d means a system or device having channels and chambers which are at the micron or submicron scale, e.g., having at least one cross-sectional dimension in a range from about 0.1 xcexcm to about 500 xcexcm.
Applications for microfluidic systems are myriad. Microfluidic systems have been proposed for capillary electrophoresis, liquid chromatography, flow injection analysis, and chemical reaction and synthesis. Microfluidic systems also have wide ranging applications in rapidly assaying compounds for their effects on various chemical, and preferably, biochemical systems. These interactions include the full range of catabolic and anabolic reactions which occur in living systems, including enzymatic, binding, signaling, and other reactions.
A variety of methods have been described to effect the transport of fluids between a pair of reservoirs within a microfluidic system or device. Incorporation of mechanical micro pumps and valves within a microfluidic device has been described to move the fluids within a microfluidic channel. The use of acoustic energy to move fluid samples within a device by the effects of acoustic streaming has been proposed, along with the use of external pumps to directly force liquids through microfluidic channels.
The capabilities and use of microfluidic systems advanced significantly with the advent of electrokinetics: the use of electrical fields (and the resulting electrokinetic forces) to move fluid materials through the channels of a microfluidic system. Electrokinetic forces have the advantages of direct control, fast response, and simplicity, and allow fluid materials to be selectively moved through a complex network of channels so as to provide a wide variety of chemical and biochemical analyses. An exemplary electrokinetic system providing variable control of electro-osmotic and/or electrophoretic forces within a fluid-containing structure is described in U.S. Pat. No. 5,965,001, the full disclosure of which is incorporated herein by reference.
Despite the above-described advancements in the field of microfluidics, as with all successes, still further improvements are desirable. For example, while electrokinetic material transport systems provide many benefits in the micro-scale movement, mixing, and aliquoting of fluids, the application of electrical fields can have detrimental effects in some instances. In the case of charged reagents, electrical fields can cause electrophoretic biasing of material volumes, e.g., highly charged materials moving to the front or back of a fluid volume. Where transporting cellular material is desired, elevated electrical fields can, in some cases, result in a perforation or electroporation of the cells, which may effect their ultimate use in the system.
To mitigate the difficulties of electrokinetic systems, simplified transport systems for time domain multiplexing of reagents has been described in WO 00/45172 (assigned to the assignee of the present invention), the full disclosure of which is incorporated herein by reference. Still further alternative fluid transport mechanisms and control methodologies to enhance the flexibility and capabilities of known microfluidic systems, including multiple modulated pressure-driven techniques, have been described in International Application No. PCT/US01/05960, the full disclosure of which is also incorporated herein by reference.
Regardless of the mechanism used to effect movement of fluid and other materials within a microfluidic channel network, accuracy and repeatability of microfluidic flows can be problematic. Quality control can be challenging in light of variability of the fluids making up these flows, and accurate control over microfluidic flows in applications such as high throughput screening would benefit significantly from stable and reliable assays. It would also be beneficial to determine additional characteristics of the fluids flowing within the microfluidic channels of a microfluidic network.
In light of the above, it would be advantageous to provide improved microfluidic devices, systems, and methods. It would be desirable if these improved techniques allowed better control over the flows within a microfluidic network, and/or increased the information provided by the microfluidic systems regarding one or more of the characteristics of the fluids flowing within a microfluidic channel of the network. It would be particularly beneficial if these enhanced techniques provided real-time and/or quality control feedback on the actual flows, ideally without relying on significantly increased system complexity or cost.
The present invention generally provides improved microfluidic devices, systems, and methods. The devices and systems of the invention generally allow the characteristics of a fluid within in a microfluidic system to be determined, often using high-throughput techniques. In many embodiments, the invention will determine the viscosity of one or more sample fluids within a microfluidic channel network of a microfluidic body. The microfluidic networks will generally include at least one flow-resisting channel segment, and viscosity may be determined by flowing the sample fluid through the channel segment, often without altering the sample viscosity by adding any detectable marker (such as fluorescent dyes or the like) to the fluid before it flows through the channel segment. These techniques can also allow the use of dyes which are not normally compatible with a particular sample fluid, for example, dyes which are not soluble or the like. The viscosity may be determined by mixing the sample fluid with a detectable marker at an intersection downstream of the flow-resisting channel segment, with the mixing characteristics at the intersection indicating the pressure drop along the channel segment (and hence the viscosity of the sample fluid). Viscosities may be determined by comparing the flow characteristics of the sample fluid with a reference fluid having a known viscosity. The sensing range may be enhanced using a plurality of flow-resisting channel segments and/or detectable fluid channel intersections.
In a first aspect, the invention provides a microfluidic viscometer system comprising a microfluidic channel network including a first flow-resisting channel segment. A sensor coupled to the first segment of the network determines a viscosity of a sample fluid therein.
In many embodiments, a body having channel walls will define the network. The network will often include a plurality of channels with one or more intersections therebetween. A flow generator coupled to the network can induce a flow of the sample fluid within the first segment. A first intersection may be in communication with the first segment, with the sensor coupled to the network at a sensor location disposed downstream of the first segment. This allows the sensor to sense a change in the flow which propagates from the first intersection to the sensor location so as to determine the viscosity of the sample fluid.
In some embodiments, the change in flow may comprise a pulse of a detectable fluid introduced at the first intersection, which may be upstream of the first segment. The system can then determine the viscosity of the sample fluid using a steady state propagation of the flow (which includes the detectable fluid pulse) from the intersection through the first segment and to the sensor location. In such embodiments, it is possible that the presence of the detectable fluid pulse may, to some extent, alter the characteristics (including the viscosity) of the sample fluid flowing through the first segment. Related embodiments may make use of a step-function change in flow of a detectable fluid.
In alternative embodiments, the first segment may be disposed upstream of the first intersection. The flow may define a ratio between a quantity of a sample fluid in the flow and a quantity of a detectable fluid in the flow, the detectable fluid being detectable by the sensor and traversing a second flow-resisting channel segment between a detectable fluid source and the intersection. By monitoring the changes in the mixing ratio, typically by monitoring the strength of a detectable signal provided from the mixed flow, a processor coupled to the sensor can determine the viscosity of the sample fluid. Advantageously, the viscosity sensing techniques of the present invention are particularly well-suited for sequential viscosity measurements of a plurality of sample fluids, particularly when the fluids are transferred along a fluid introduction channel in the form of a capillary extending or protruding from the microfluidic body.
In a method aspect, the invention comprises determining a viscosity of a sample fluid. The method comprises altering a flow of a flow-restricting microfluidic channel segment. The viscosity of the sample fluid may be determined by monitoring the altered flow.
In many embodiments, a first flow of a reference fluid through the flow-resisting channel will be monitored, the reference fluid having a known viscosity. A second flow through the flow-resisting channel will also be monitored, the second flow comprising the sample fluid. The viscosity of the sample fluid may be determined at least in part by comparing the first and second flows, with calculations based in part on the known viscosity of the reference fluid. The first and second flows may be monitored by a sensor disposed downstream of the flow-resisting channel with an intersection disposed between the flow resisting channel and the sensor. The flows can be monitored by sensing a ratio of the sample fluid to a detectable fluid. Advantageously, a plurality of sample fluids can be sequentially transferred to the flow-resisting channel segment, allowing the viscosities of the samples to be determined in a high-throughput manner. In some embodiments, fluids may be dispensed and/or mixed within a microfluidic network, for example, allowing viscosities of fluid mixtures to be determined as a function of their composition.
In another aspect, the invention comprises a microfluidic channel network including a first flow-resisting channel segment. A sensor is coupled to the network for sensing flows through the first segment. A processor is coupled to the sensor. The processor derives a viscosity of a sample fluid by comparing first and second flows through the first segment.
The system may further include a reference fluid disposed within the network. The first flow may comprise the reference fluid, and the second flow may comprise the sample fluid. In many embodiments, the second flow within the first segment may be substantially composed of the sample fluid. The processor may calculate the viscosity of the sample fluid based at least in part on a viscosity of the reference fluid.
In many embodiments, a second flow-resisting channel segment will be coupled to the first segment at a first intersection. A first detectable fluid may be disposed within the second segment. The first intersection can be downstream of the first segment, and the sensor can monitor the flow through the first segment by sensing a quantity of the first detectable fluid added to the flow at the first intersection. Still further additional flow resisting channel segments may be coupled to the first segment by additional intersections. The intersections may be separated by associated flow-resisting channel segments, and the sensor may monitor the flow by sensing a quantity of the first detectable fluid added to the flow at the intersection. Alternatively, one or more additional flow-resisting channel segments may be couple to the first segment, with the sensor monitoring the flow through the first segment by sensing a quantity of a second detectable fluid added to the flow through the third segment. In such embodiments, the second and third segments may have differing resistances to flows therein. The first and second detectable fluids may be independently detectable by the sensor, for example, comprising dyes having differing color signatures.
The first segment may comprise a channel having a locally enhanced resistance to flows therein. For example, the channel region may have a reduced cross-sectional dimension, such as a reduced depth, a reduced width, or the like. Alternatively, a flow occluding structure may be disposed within the channel.
The first flow may comprise a reference fluid having a known viscosity, and the second flow may comprise a combination of the sample fluid and a detectable fluid. This combination can define a ratio, with the processor identifying the ratio from a signal produced by the sensor. For example, the sensor may sense a light signal generated by a fluorescent dye of the detectable fluid, with a relative strength of the fluorescence indicating the ratio.
In many embodiments, the processor may derive the viscosity of the sample fluid by determining a rate of change of a signal generated by the sensor. In some embodiments, the processor may derive the viscosity of the sample fluid by determining a magnitude of a change of a signal generated by the sensor. The processor may determine the sample viscosity throughout a range of at least about two orders of magnitude of cp units, preferably through a range of at least three orders of magnitude of cp units. The processor may determine the sample viscosity throughout at least a range from about 1 cp to 100 cp, preferably from about 1 cp to 1000 cp, and optionally from 0.1 cp to 1000 cp. By, for example, simply altering driving pressures, larger viscosities may be sensed, i.e., 1,000 cp-100,000 cp. Hence, at least viscosities throughout ranges of at least 2 or 3 orders of magnitude may be sensed.
A microfluidic system may optionally include a sample fluid source which includes a plurality of sample fluids and a sample fluid introduction channel. The sample fluids may then be sequentially transferable along the fluid introduction channel to the flow resisting channel so as to sequentially determine viscosities of the sample fluid. The sample introduction channel may comprise a capillary extending from the microfluidic body, with the capillary being extendable sequentially into the sample fluids. Generally, the capillary will have significantly less resistance to flow than the first segment.
In yet another embodiment, the invention provides a microfluidic system comprising a microfluidic body having a network of channels. A flow generator induces a flow within the network, and a sensor transmits a signal indicating a time of the flow. A processor effects feedback control of the flow in response to the time signal. Optionally, the processor may determine a viscosity for use in the feedback control loop.
Yet another embodiment of the invention provides a microfluidic system comprising first and second immisciable fluids. A microfluidic body having a network of channels combines the fluids therein, and a sensor is coupled to the network so as to define a viscometer. The viscometer measures interfacial properties of the combined fluid.